FIG. 1 shows a schematic diagram of a traditional photoelectric conversion device 1 and a photoelectric conversion method thereof. As shown in FIG. 1, the photoelectric conversion device 1 includes a P-type semiconductor layer 11 and an N-type semiconductor layer 12.
The P-type semiconductor layer 11 has a first valence band 111, a first conduction band 112, and a first bandgap 113. The N-type semiconductor layer 12 has a second valence band 121, a second conduction band 122, and a second bandgap 123. A depletion zone 13 is formed on the PN junction between the P-type semiconductor layer 11 and the N-type semiconductor layer 12. An internal electric field is created in the depletion zone 13.
A first potential slope 131 is formed between the first valence band 111 and the second valence band 121, and all three are below the Fermi level 133. A second potential slope 132 is formed between the first conduction band 112 and the second conduction band 122, and all three are above the Fermi level 133.
When the photoelectric conversion device 1 absorbs a plurality of photons 14 and produces electron-hole pairs such as a first electron 141a and a first hole 141b, and a second electron 142a and a second hole 142b, the first electron 141a may transition from the first valence band 111 to the first conduction band 112, and the second electron 142a may transition from the second valence band 121 to the second conduction band 122.
Then, owing to the diffusion effect, the first electron 141a and the second electron 142a may arrive on the second potential slope 132 of the depletion zone 13, and the first hole 141b and the second hole 142b may arrive below the first potential slope 131 of the depletion zone 13. Next, with the internal electric field in the depletion zone 13, the first electron 141a, the second electron 142a, the first hole 141b and the second hole 142b are separately transferred to an external circuit 15, thereby creating electrical energy.
One disadvantage of such conventional photoelectric conversion device is that both the P-type semiconductor layer and the N-type semiconductor layer have bandgaps. For example, the band gap of a P-type semiconductor layer or an N-type semiconductor layer made of silicon (Si) is about 1.1 eV (electron volts). As a result, the light absorption range of the photoelectric conversion device is restricted by the bandgaps, such that some photons cannot be absorbed by the photoelectric conversion device, resulting in a reduction in the number of photons being absorbed, and failure in producing a large amount of electrons and holes.
Furthermore, the electrons and holes are conducted externally at a lower rate with a lower capture, resulting in high energy loss, smaller voltage and current, and poorer photoelectric conversion efficiency. As a result, the photoelectric conversion device can only obtain a small number of low-energy electrons and holes (cold carrier), and produces electricity of a low voltage and a low current.
Therefore, there is a need to develop a photoelectric conversion device and a method thereof to overcome the above problems.